1. Field of the Invention
This invention relates generally to circuit breakers having a magnetic trip assembly in which the magnetic field induced by an abnormal current unlatches a latchable operating mechanism to trip the breaker, and more particularly to such a magnetic trip assembly which includes a supplemental magnetic core which allows the magnetic trip assembly to trip the breaker at relatively low levels of overcurrent.
2. Background Information
Circuit breakers provide protection for electrical systems from electrical fault conditions such as current overloads and short circuits. A common type of circuit breaker used to interrupt abnormal conditions in an electrical system incorporates a thermal trip device which responds to persistent low levels of overcurrent and a magnetic trip assembly which responds to higher levels of overcurrent in a fraction of a second. An example of such a circuit breaker is disclosed in U.S. Pat. No. 4,528,531. In such circuit breakers, the thermal trip device comprises a bimetal which bends in response to the persistent low level overcurrent passed through it to unlatch a latchable operating mechanism. The latchable operating mechanism is spring operated to open electrical contacts which interrupt the current. The magnetic trip assembly includes an armature which is spring biased to latch the operating mechanism. The current through the bimetal produces a magnetic field which is concentrated by a magnetic yoke to attract the armature and unlatch the operating mechanism at a specified level of overcurrent. The bimetal in these circuit breakers acts as a one turn electromagnet for the magnetic trip assembly.
Such circuit breakers have been in use for many years and their design has been refined to provide an effective, reliable circuit breaker which can be easily and economically manufactured on a large scale.
Recently there has developed a market for such circuit breakers with a magnetic trip assembly which operates at lower levels of instantaneous overcurrent. The level of overcurrent at which the magnetic trip operates is a function of several factors, including the friction force on the spring operated latchable operating mechanism, the spring constant of the spring biasing the armature to latch the operating mechanism, the magnitude of the magnetic field produced by the overcurrent and the coupling of the magnetic field to the armature.
In previous designs, the magnetic trip mechanism would operate at generally fifteen times (15X) the breaker rating. More recently, the market has demanded breakers which have a magnetic trip rating in the range of 5X to 10X. However, to the present, a device has not been developed which is readily adaptable to existing breaker designs.
There remains a need for an improved circuit breaker with a magnetic trip assembly for use in a multiphase system which becomes operative at lower current levels for example, having a rating of 5-10X, but which can also be adapted for use in a single phase breaker.
There is a further need for such a circuit breaker which can be produced economically.
There is a related need for a circuit breaker with a low magnetic trip which requires little modification to the existing single phase and multiphase circuit breaker designs.